Power looms have so-called shedding systems, which serve to move warp yarns upward or downward out of the warp yarn plane in order to form a so-called shed so that a weft yarn can be inserted. The weft yarn insertion systems, which employ water or air, for instance, have a power potential for maximum weaving speeds. As a rule, however, this potential cannot be fully exploited, because the existing shedding systems cannot withstand the loads that result from an overly high operating speed. The loads result from the accelerations in the up-and-down motion of the shafts with which the heddles are retained. The motion is generated by so-called eccentric machines or shaft machines. Although the most harmonic possible motions are sought here and achieved, nevertheless vibration occurs in the shedding system and the associated mechanism that connects the heddle shafts with the eccentric machines. This vibration puts a load on all the elements of the shedding system and leads to premature wear or breakage of components. Heddle breakage, warp yarn breakage, and the resultant down times of the machines are the result of such excessive loads.
Various concepts with a view to reducing wear in the shedding system and reducing vibration have been developed:
For instance, from Swiss Patent Disclosure CH 558 435, a heddle shaft drive mechanism is known that includes a rod linkage disposed between the heddle shaft and a shaft mechanism.
The rod linkage includes a strap assembly with a built-in shock absorber. In one variant embodiment, the shock absorber may be embodied as a rubber block. It then connects two rigid halves of the strap assembly that extend away from it.
Such a rubber block achieves adequate oscillation damping only if it has considerable axial resilience, which is disadvantageous for the precision of shaft motion. Moreover, it is an additional mass that must be moved and that in cooperation with further elements, such as connections that have play, may again be a source of vibration.
From Utility Model 7832785, a rod linkage for driving a heddle shaft is also known, in which bell crank levers disposed below the shaft are connected to the shaft via thrust rods. In the upper eyelet or joint of each thrust rod, elastic elements are provided, in the form of vulcanized bodies. The damping elements are thus located at the output of the rod linkage that connects the shaft machine to the heddle shaft.
From German Utility Model DE 296 11 305, a device for oscillation damping of heddle shafts is known in which oscillation damping devices are disposed on the guide face, toward the warp beam, of guide elements of the heddle shaft. These devices are formed by a soft rubber plate, for instance. A guide piece provided on the heddle shaft extends along the soft rubber plate, and as a result, oscillations of the heddle shafts that extend in the direction of the warp yarns can be damped.
From Swiss Patent 549 668, a rod linkage for driving a heddle shaft is known which has spring joints instead of conventional hinge joints. These spring joints are formed by leaf springs or rubber blocks. This provision serves to reduce the wear that otherwise occurs at the joints. Moreover, the goal is to largely avoid the necessity of lubricating joints that move slowly back and forth.
From European Patent Disclosure EP 0 870 856 A1, a rod linkage provided for driving a heddle shaft is known that is connected to the shaft drive via a cushioned strap assembly. To that end, the strap assembly is divided into two parts, between which a compression spring assembly is operative.
The resilience of such a spring assembly may be unwanted.